Sketchpad 3.5 is essential if you want to work with images while offline.
Sketchpad offers the essentials, like drawing freehand or arranging geometric figures, just enough for sketching a map or converting among image formats. It has an easy-to-use pallete for selecting colors as well as the ability to add textures and gradients. I was able to doodle with the Chromebook's touchpad, but it works much better with a touch-screen system, like the Acer C720P.
The app has a variety of drawing styles, including crayon, pencil and spray paint; you can also add text boxes. I really like the eraser and the ability to undo mistakes.
When done, the drawing can be exported as a PDF or the image can be compressed in a Zip file.